Time-resolved PIV measurement of influence of upstream roughness on separated and reattached turbulent flows over a forward-facing step
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Bibliographic record
Abstract
This study reports an experimental investigation of the effects of upstream roughness on a low Reynolds number turbulent boundary layer over a forward-facing step. Two types of upstream roughness were investigated, including a transitionally rough 16-grit sandpaper (ks+ ≈ 69) and fully rough staggered cubes (ks+ ≈ 500). A two-dimensional two-component time-resolved particle image velocimetry (2D-2C TR-PIV) method was used to measure the time-averaged mean velocities, Reynolds stresses, temporal auto-correlations and frequency spectra of the flow field to quantify the influence of upstream roughness on the downstream evolution of the turbulence over the step. The results indicate that upstream roughness decreased the vortex shedding frequency. Roughness also decreased the reattachment length by enhancing the streamwise turbulence intensity level, reducing the magnitude of backflow and suppressing the vortex shedding frequency in comparison to the smooth wall. In the recirculation region, upstream roughness reduced the mean streamwise velocity only in the outer layer. The Reynolds stresses remained relatively unchanged by the sandpaper roughness but were significantly modified by the cube roughness. Downstream of the leading edge, the staggered cubes increased the streamwise Reynolds stress both near the wall and outside the shear layer but decreased the wall-normal Reynolds stress and Reynolds shear stress within the shear layer. These modifications are inversely proportional to distance in the recirculation region. The life times of the streamwise and wall-normal velocity fluctuations increase with streamwise distance and are much longer in the redevelopment region than in the recirculation region. Quadrant decomposition and joint probability density functions of the velocity fluctuations were also measured to characterize upstream roughness effects on the downstream evolution of the dominant motions producing the Reynolds shear stress.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it